This invention is intended as an improvement to the "Differential Harmonic Drive" disclosed in co-pending U.S. application Ser. No. 427,634, filed Dec. 26, 1973, by Pieter F. Hoos, now U.S. Pat. No. 3,868,867 granted Mar. 4, 1975. In that system the reference sprocket is stationary and the power drive operates as here proposed. However, for manual drive, the system was simply inactivated and a handwheel was coupled to the output, so that there was a direct drive, with no mechanical advantage.